783620H / 1013 5. MENU COMMANDS
Choosing a ZDT Mode
When the counting rate is essentially constant during the time required to acquire the spectrum, the standard mode — ZDT Off — is the preferred mode; only the uncorrected spectrum is collected and stored in the spectrum file. But, if the counting rate varies significantly during the measurement time, the standard mode will not yield the proper dead-time-corrected counting rate. This can be most easily understood by noting that the uncorrected mode compensates for dead-time losses by extending the real counting time. Hence a sample containing both a shortlived high-activity isotope and a long-lifetime lower-activity isotope will experience very high dead-time losses during the first few seconds of the measurement, as the short-lifetime isotope decays rapidly. This high dead time will cause the counting time to be extended after the shortlived isotope has decayed to zero activity, and the system will count the low-activity isotope for the extra time. Consequently, the average activity of the short-lived isotope will be underestimated.
If you anticipate significantly varying counting rates during the time taken to acquire the spectrum, the CORR_ERR ZDT mode should be used. The CORR_ERR mode corrects for deadtime losses over minuscule time intervals by adding counts to the ZDT spectrum in proportion to the instantaneous ratio of real time to live time. Thus, the dead-time correction can correctly track rapidly changing counting rates. The CORR_ERR mode should be used whenever the counting rate may change significantly during the measurement time. In addition to the rapidly decaying isotope example above, the CORR_ERR mode should be used when monitoring cooling water flow from a nuclear reactor. The CORR_ERR mode accommodates brief bursts of high-activity in the water flowing past the gamma-ray detector. Both the corrected and error spectra are stored in the resulting spectrum file.
Note that the counts in the ZDT spectrum must be divided by the elapsed REAL time to compute the dead-time corrected counting rate. It is important to note that the standard deviation in the
N
ZDT
counts in a gamma-ray peak in the ZDT spectrum is not . Instead the standard deviation is obtained from the N
ERR
counts in the same peak ROI in the accompanying error spectrum.
The standard deviation in this case is . And the standard deviation in the computed counting rate, , is .
The NORM_CORR Diagnostic Mode
Why is there a NORM_CORR mode, and why should you avoid using it? This mode simultaneously collects the ZDT spectrum and the conventional uncorrected spectrum. It is useful for demonstrating that the counts in the uncorrected spectrum divided by the live time is the same counting rate as the counts in the ZDT spectrum divided by the real time, in the special case of constant counting rate. Because the error spectrum is not collected in NORM_CORR mode, the standard deviation in the ZDT counts cannot be calculated if the counting rate is varying.
97
